Although in past times it has been known to provide signalling on a power line to a user's equipment, such as in the customary standard telephone system wherein a voice signal is combined with a power signal, or to provide signalling over mains supply lines, traditional systems do not provide sufficient bandwidth for modern data communications generally and packet-based communication systems such as Ethernet. ISDN communication systems include the transmission of power and digital data between an ISDN modem and a telephone switch. However, ISDN data rates are very much lower than typical data rates in packet-based communication systems such as Ethernet. It is common practice therefore to provide power and data over separate lines to network devices such as data terminal equipment. Nevertheless, the provision of power to an end device, such as a computer, Ethernet telephone, or other peripheral device, using standard Ethernet physical connection, such as twisted-pair wiring, is desirable for many purposes. The local provision of power can be problematic for Ethernet based devices. One example is a security camera wherein the provision of local continuous power may be difficult or undesirable, from the point of view of making the camera secure, and yet the provision of battery power, with its inherently limited life, may be undesirable or inconvenient. Furthermore, more recently developed wireless network adapters would be more versatile if they allow associated terminal equipment, such as personal computers, to be moved to any location within the range of the communication system without requiring a neighbouring mains power outlet.
It is currently known to provide power over an Ethernet connection and specific devices for the provision of power over Ethernet connections are described in, for example, U.S. Pat. No. 5,994,998 issued 30 Nov. 1999 to Fisher et al and U.S. Pat. No. 6,535,983 issued 18 Mar. 2003 to McCormack et al, both assigned to 3Com Corporation of Santa Clara, Calif.
It is known in the art, from the aforementioned patents and elsewhere, particularly a recently developed draft Standard IEEE 802.3af, to provide power from an Ethernet port over a twisted-pair link by means of balanced transformer couplings and appropriate filtration. The current draft Standard allows power up to a specified limit, currently 15.4 watts, to be sourced from a ‘power over Ethernet’ (PoE) port to a powered device. At least some of the ports of a network device, such as a switch, are coupled to a power source, herein called power supply equipment (PSE) which is controllable to supply power up to that limit to a multiplicity of ports. However, for PSEs that are not capable of supplying the maximum power simultaneously to all the respective ports, due for example to power supply design constraints, there inherently exist circumstances wherein ‘power over Ethernet’ ports must restrict or cease the supply of power. This may happen dynamically as existing powered devices fluctuate in their power requirements or as new powered devices are connected to the power supply equipment. This can lead to unfortunate occurrences, such as an unexpected cessation of supply to a particular powered device.